Thermally sprayable ceramics

ABSTRACT

Cordierite coatings capable of withstanding severe mechanical and thermal conditions. Methods of applying such coatings and components protected thereby.

This application is a continuation of application Ser. No. 294,250 filedAug. 18, 1981 (now abandoned).

This invention relates to ceramic compounds and, more particularly, tofused ceramics in powdered form for thermal spray (flame and plasma)applications in order to obtain a controllable high porosity body foruse as, for example, a thermal barrier, abradable seal.

It is well known that certain components in many industries are subjectto extreme thermo-mechanical conditions. These conditions can give riseto failure of the components or restrict seriously the operationalregimes of the components. For example, in gas turbine engines, hightemperatures and thermal shock conditions must be carefully consideredin systems design and materials specification. Parts must be protectedfrom severe thermal and mechanical conditions, this frequently beingaccomplished by protective coatings. In gas turbine engines, forexample, the wall of the shroud frequently comes in contact with therotating turbine blade. It is desirable to create a gas path sealbetween the blade and the shroud, restricting gas leakage and permittinghigher operating gas pressures and temperatures. Such sealing willincrease operating efficiency.

The shroud wall, operating under severe mechanical (rubbing and erosion)and thermal conditions, must be protected, yet at the same time be ableto accommodate the cutting action of the blade tip, which processaffects a mechanical seal. If the seal thus formed is not properlyabradable (i.e., the rubbing action causes material failure andspallation) or of insufficient thickness, the seal will be lost and/orthe blades may be damaged, risking engine failure.

Abradable gas path seals for gas turbine engines are expected to berequired to operate at increasing temperatures. High performance needswill include abradability and erosion resistance at temperaturesapproaching 2400° F. (1316° C.) with the seal material having lowthermal conductivity to enable it to also act as a thermal barrier.Thermally sprayed oxides are excellent candidate materials for thisneed.

To endow the oxide with acceptable abradability, the seal materialshould be porous. The porosity also decreases thermal conductivity,enabling the system to operate at increased temperatures.

To enhance adhesion and mechanical performance it may be advisable toemploy a graded coating (either continuously graded or layered),enabling the system to sustain residual tensile stresses generatedthrough fabrication and arising from thermal cycling. Such grading witha high temperature alloy (e.g., CoCrAlY) thus permits higher performancebehavior.

The above stated needs, as well as economic considerations offabrication and repair procedures, indicate thermal spraying as thecoating process of choice. Both combustion gun flame spraying and plasmaspraying can be employed. They endow the oxide coating with considerableporosity and are simple and straightforward.

Currently, metal-polymer composites are flame sprayed on shrouds tofurnish a seal. The resulting porous metal (the polymer is thought to be"burned-out" during the spraying process) gives a low temperature seal.Metal-graphite has been used as well. In order to increase operatingtemperature, however, refractory ceramics are required.

We have invented, and disclose herein, a new class of materials whichsatisfies all of the technical, processing, and economic needs forthermal barrier gas path seals. These novel materials are thermallysprayable powders formed from fully fused cordierite oxides. Thismaterial, when thermally sprayed by flame or plasma, yields a highlyporous, highly thermal shock resistant, low thermal conductivity, welladhered oxide having excellent abradable and erosion resistantproperties.

According to one feature of the invention, there is also provided amethod of applying, in some cases and without a metal bond coat, a highporosity thermal barrier coating to a machine component by thermallyspraying fused cordierite powder onto the component until a coating ofdesired thickness has been formed. The coating may be applied using anysuitable thermal spray gun. Examples include combustion and plasma gunsmanufactured and/or marketed by Dresser Industries, Plasmadyne,Plasmatechnic, Metco, Electroplasma, and Castolin-Eutectic.

The fused cordierite may be applied at commercial rates and has beenexperimentally applied at three pounds per hour. Higher rates may beused depending upon the equipment and application. The sprayingparameters are determined by the exact composition, particle size andretention time.

One exemplary material we have employed is in the cordierite-mullitefamily and has a typical composition as follows.

    ______________________________________                                        (2MgO.5SiO.sub.2.2Al.sub.2 O.sub.3)                                           ______________________________________                                               MgO   13.7% wt.                                                               Al.sub.2 O.sub.3                                                                    34.9% wt.                                                               SiO.sub.2                                                                           51.4% wt.                                                        ______________________________________                                    

The melting temperature for the eutectic cordierite is 1470°±5° C.

The cordierite may be formed in a variety of known ways using acombination of materials such as: (i) clay, kyanite, pyrophillite,quartz, or silica; (ii) talc or the like; and (iii) alumina or aluminumhydroxide. Appropriate techniques for making our novel compositions fromthe foregoing raw materials will be apparent to those skilled in therelevant arts from, for example, U.S. Pat No. 3,950,175 issued Apr. 17,1976, to Lachman et al which is hereby incorporated herein by reference.

Additives such as yttria, or yttrium, hafnium and other rare earths ortheir oxides may be added to the composition to improve the corrosionresistance, strength, and other properties of the coating. Typically, arare earth additive such as yttrium will be added in amounts notexceeding four percent of the composition by weight. In fact, two weightpercent may in many cases prove to be the practical maximum limit withone-half percent perhaps most often being preferred.

Also, mixtures of rare earths with refractory oxides such as zirconiacan at times be employed to advantage in particular applications of ourinvention.

In cases where a mixture of additives is employed, on the other hand,larger amounts will typically be used. In the case of theyttria/zirconia combination of additives, for example, as much as tenweight percent of additive may be employed. Up to four weight percent ofyttria can be used in this case with the balance being zirconia.

It is much preferred that the material have a uniform meltingtemperature and that all particles of the material have the samecomposition. These goals may be achieved by making up a cordieriteforming mixture and then melting or fusing it before the material ismelted in the thermal spray application step. This has the effect ofremoving low temperature phases which would otherwise mar or harm theproperties of the coating to be formed. The material may be pre-reactedor fused using standard ceramic processes normally used to form thecordierite.

Additives such as yttria, hafnia, or various other rare earths may beincluded in the fused cordierite to modify the corrosion resistance,strength, adhesion, or other chemical and thermo-mechanical propertiesof the coating.

According to another feature of our invention, it is not absolutelynecessary to precoat the substrate in many applications, but precoatingmay be done to improve its application for extremely corrosiveenvironments. It is possible to use the material as a precoat for otherceramics because of its excellent adhesion to metal and itscompatibility with other ceramics.

The thickness of our novel coatings can exceed other thermal sprayapplied ceramics with thicknesses exceeding 2 mm without loss ofadhesion strength. The coating displays an excellent interface, givinggood adhesion to the substrate.

An additional important and distinguishing feature of our novel productis its high potential for a significant exothermic release of energy.This is associated with recrystallization, the threshold temperature ofwhich appears to be in excess of 850° C. and which will occur duringthermal spraying and on the heating of the thus formed coating. Thisexothermic reaction is unique in thermal spray applied oxide ceramicsand is important because it enhances cohesion and adhesive bondstrengths.

Also exemplary of the applications in which the compositions disclosedherein can be employed to advantage as precoats are those in which thecoated component is exposed to an erosive environment. In this case,erosion resistance can be significantly improved by first precoating thecomponent being protected with a coating material as described hereinand then superposing a coating of zirconia (which has excellent erosionresistance but inferior adhesion strength to metallic substrates).Because our novel coatings adhere tightly to metals and are alsocompatible with ceramics, therefore also adhering tightly to them, theyfurnish excellent interfaces between the metallic substrate and theerosion resistant zirconia.

Another previously mentioned way in which our novel coating compositionscan be employed to advantage in environments such as those justdiscussed, for example, is in the form of graded coatings; i.e.,coatings in which that portion adjoining the metallic substrate is ofthe character described above and in which the composition changes fromthat to one of a different character (for example, essentially zirconia)at the exposed surface of the coating. This can be accomplished eitherby applying superposed discrete coatings of different compositions or bycontinuously varying the composition of the coating over its thickness.Both techniques are well developed, and either can be employed.

According to another aspect of the invention, there is provided amachine component having thereon a coating of a material comprising acordierite.

The coating is characterized by outstanding thermal shock resistance andexcellent thermal barrier properties. The coating has a controllableporosity range of up to 40 volume percent, which is an unusually highlevel, in that other materials do not provide reliable porosities muchin excess of 25 volume percent.

Porosity is important for the reasons discussed above. That parameter ismeasured by sectioning the coating and measuring the percentage of thesurface of the exposed face which is made up of pores.

Organic and inorganic additives may be included to fill some or all ofthe pores to modify the properties of our novel coatings, depending uponthe temperature of utilization.

For example, pore filling additives may be employed to keep corrosivematerial in the ambient environment from penetrating to the coatedsubstrate or to keep cracks from developing in the coating. Or, thepores may be filled with an appropriate lubricant such as Teflon,molybdenum disulfide, or graphite to improve lubricity in applicationswhere the component functions as a bearing or is otherwise in movingcontact with another component.

The porosity of the coating material can be varied in several ways;e.g., by varying the spray rate and spray temperature. Also, porositymay be controlled, if a plasma arc gun is employed, by regulating thevoltage across, and/or current between, the electrodes of the plasma arcgun (this is a common capability of those devices).

Porosity may also be controlled by varying the distance between thethermal spray gun and the target or component being coated. As thedistance between the gun and the target is increased, the coating willbecome more porous.

In addition to having the desirable properties identified above, thenovel coating materials we have invented are useful because of theirvibration and sound absorbing properties. Consequently, the emphasis onthermal properties is not intended to limit the scope of our inventionor the applications of that invention intended to be encompassed in theappended claims.

The machine component to which our coating materials are applied may bepart of a rocket engine, an aircraft jet engine, an abradable seal, anaircraft carrier structural member, or any other application requiring athermal barrier coating.

One important object of our invention resides in the provision of novel,improved ceramic compositions.

Related, but more specific objects of our invention reside in theprovision of compositions in accord with the preceding object:

which are tightly adherable to metallic surfaces;

which are porous and abradable but erosion resistant;

which are temperature resistant and have low thermal conductivity;

which can be thermally sprayed;

which are resistant to thermal shock and otherwise have excellentmechanical properties;

which are members of the cordierite-mullite family;

which, in conjunction with the preceding object, contain rare earthmetal and/or other additives capable of improving such properties of thecomposition as strength and corrosion resistance.

Yet another important and primary object of our invention resides in theprovision of techniques for applying the novel compositions disclosedherein.

A related, also important but more specific, object of the inventionresides in the novel use of thermal spraying techniques for applying theaforesaid compositions.

Still another important and primary object of our invention resides inthe provision of novel, improved methods of protecting machinecomponents against severe thermal and mechanical conditions by coatingthem with compositions as disclosed herein and characterized in thepreceding objects.

Certain primary objects of our invention have been identified above.Other important objects and novel features of that invention will becomeapparent from the appended claims, from the illustrative example of howour invention be can be applied that follows, and from the drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE of the drawing is a phase diagram showing in the areaencircled by a phantom line the full range of the ceramic compositionswe employ in the coatings we have invented and disclosed herein.

Fused cordierite, made by melting the raw materials identified above inthe specified proportions in a fusion furnace and reducing the resultingsolid composition to a -61 to +11 microns powder, was sprayed at a rateof 3 lbs/hour through a Metco 3MB gun onto a steel substrate to athickness in excess of 2 mm.

The coating thus formed was found to have the following properties:

    ______________________________________                                                Profile microhardness                                                                           600-700 at 15 gram                                                            load                                                Superficial                                                                           Rockwell          R.sub.B 70                                          hardness                                                                      Metallo-                                                                              Microscope        Lamillar structure;                                 graph                     interlamillar porosity                              Porosity                                                                              By optical metallograph                                                                         20%-40%; porosity;                                                            mainly discontinuous                                Thermal 12 times .25 mm from                                                                            No spalling, peeling                                test    nozzle of 35 KW argon-                                                                          or cracking                                                 hydrogen plasma. Heated                                                       till substrate was red-                                                       white hot - then water                                                        quenched                                                              Thickness                                                                             Continuous overcoating                                                                          2.5 mm (not the limit                                                         of thickness)                                       Bond test                                                                             Substrate - 10 × 10 × 2 mm                                                          No interfacial failure                                                        to angles greater than                                                        90° (approximately                                                     120°)                                        Crystal-                                                                              X-Ray powder diffraction                                                                        As sprayed structure                                linity  methods           is amorphous; crystal-                                                        lization temperature                                                          appears to be in ex-                                                          cess of 850° C.                              ______________________________________                                    

The invention may be embodied in other specific forms without departingfrom the spirit or essential characteristics thereof. The presentembodiments are therefore to be considered in all respects asillustrative and not restrictive, the scope of the invention beingindicated by the appended claims rather than by the foregoingdescription; and all changes which come within the meaning and range ofequivalency of the claims are therefore intended to be embraced therein.

What is claimed and desired to be secured by Letters Patent is:
 1. Acomponent comprised of a substrate which has a metallic surface andwhich is susceptible to abrasion and erosion at high temperatures and aporous, abradable, erosion, shock and temperature resistant, tripleoxide coating which has low thermal conductivity tightly bonded to saidmetallic surface of said substrate, said coating being, at least inpart, a reaction product of a fully fused, thermally sprayablecordierite coating precursor which is capable of releasing sufficientexothermic energy when heated to promote adhesion between the coatingand the substrate.
 2. A component comprised of a substrate which has ametallic surface and a coating which is capable of absorbing soundand/or other vibrations tightly bonded to said metallic surface of saidsubstrate, said coating being, at least in part, a reaction product of afully fused, thermally sprayable cordierite coating precursor which iscapable of releasing sufficient exothermic energy when heated to promotethe adhesion of the coating to the substrate.
 3. A component as definedin claim 1 or in claim 2 wherein said fully fused, thermally sprayablecordierite coating precursor is essentially a single phase compositionin the cordierite-mullite family.
 4. A component as defined in claim 1or in claim 2 wherein said fully fused, thermally sprayable cordieritecoating precursor is obtained by melting together a mixture of magnesiumoxide, aluminum oxide, and silicon oxide, said precursor having acomposition bounded by the phantom line in the drawing.
 5. A componentas defined in claim 1 or in claim 2 in which the coating consistsessentially of the reaction product of a fully fused, thermallysprayable coating precursor as aforesaid.
 6. A component as defined inclaim 1 or in claim 2 wherein the cordierite constituent of said coatingprecursor has the formula 2MgO5SiO₂ 2Al₂ O₃.
 7. A component comprised ofa substrate which is susceptible to abrasion and erosion at hightemperatures and a porous, abradable, erosion, shock and temperatureresistant, triple oxide coating which has low thermal conductivitytightly bonded to said substrate, said coating being, at least in part,a reaction product of a fused, thermally sprayable cordierite coatingprecursor which is capable of undergoing an exothermic reaction whenheated, said coating also including a rare earth metal or rare earthmetal oxide in an amount effective to favorably influence a desirableproperty of the coating such as adhesion, strength, or corrosionresistance.
 8. A component comprised of a substrate and a coating whichis capable of absorbing sound and/or other vibrations tightly bonded tosaid substrate, said coating being, at least in part, a reaction productof a fused, thermally sprayable cordierite coating precursor which iscapable of undergoing an exothermic reaction when heated, said coatingalso including a rare earth metal or rare earth metal oxide in an amounteffective to favorably influence a desirable property of the coatingsuch as adhesion, strength or corrosion resistance.
 9. A componentcomprised of a substrate which is susceptible to abrasion and erosion athigh temperatures and a porous, abradable, erosion, shock andtemperature resistant, triple oxide coating which has low thermalconductivity tightly bonded to said substrate, said coating being, atleast in part, a reaction product of a fused, thermally sprayablecordierite coating precursor which is capable of undergoing anexothermic reaction when heated, said coating also including thecombination of a rare earth metal or rare earth metal oxide and arefractory oxide.
 10. A component comprised of a substrate and a coatingwhich is capable of absorbing sound and/or other vibrations tightlybonded to said substrate, said coating being, at least in part, areaction product of a fused, thermally sprayable cordierite coatingprecursor which is capable of undergoing an exothermic reaction whenheated, said coating also including the combination of a rare earthmetal or rare earth metal oxide and a refractory oxide.
 11. A componentas defined in any one of claims 7-10 wherein the rare earth metalconstituent is yttrium or hafnium or an oxide of yttrium or hafnium. 12.A component comprised of a substrate which is susceptible to abrasionand erosion at high temperatures and a porous, abradable, shock andtemperature resistant, triple oxide coating which has low thermalconductivity tightly bonded to said substrate, said coating being, atleast in part, a reaction product of a fused, thermally sprayablecordierite coating precursor which is capable of undergoing anexothermic reaction when heated, at least the pores of the cordieriteconstituent of said coating being filled with a substance which iscapable of keeping corrosive material in the ambient environment frompenetrating to the coating substrate or a substance which is capable ofkeeping cracks from developing in the coating or with a lubricant.
 13. Acomponent comprised of a substrate and a coating which is capable ofabsorbing sound and/or other vibrations tightly bonded to saidsubstrate, said coating being, at least in part, a reaction product of afused, thermally sprayable cordierite coating precursor which is capableof undergoing an exothermaic reaction when heated, at least the pores ofthe cordierite consittuent of said coating being filled with a substancewhich is capable of keeping corrosive material in the ambientenvironment from penetrating to the coating substrate or with asubstance which is capable of keeping cracks from developing in thecoating or with a lubricant.
 14. A component comprised of a substratewhich is susceptible to abrasion and erosion at high temperatures; aporous, abradable, erosion, shock and temperature resistant, tripleoxide coating which has low thermal conductivity tightly bonded to saidsubstrate, said coating being, at least in part, a reaction product of afused, thermally sprayable cordierite coating precursor which is capableof undergoing an exothermic reaction when heated; and a layer of anerosion resistant refractory oxide overlying and bonded to saidsubstrate, said coating being, at least in part, a reaction product of afused, thermally sprayable cordierite coating precursor which is capableof undergoing an exothermic reaction when heated; and a layer of anerosion resistant refractory oxide overlying and bonded to said coating.15. A component comprised of a substrate; a coating which is capable ofabsorbing sound and/or other vibrations tightly bonded to saidsubstrate, said coating being, at least in part, a reaction product of afused, thermally sprayable cordierite coating precursor which is capableof undergoing an exothermic reaction when heated; and a layer of anerosion resistant refractory oxide overlying and bonded to said coating.16. A component as defined in claim 14 or in claim 15 wherein saidrefractory oxide is zirconia.